With the progressing of the semiconductor integrated circuits to the ULSI (ultra large scale integration) level or even higher level, the integrity of the integrated circuits rises with an explosive rate. The capacity of a single semiconductor chip increases from several thousand devices to hundreds of million devices, or even billions of devices. Taking DRAM (dynamic random access memories) for example, the increasing integrity in manufacturing extends the capacity of a single chip to step from earlier 4 megabit to 16 megabit, and further to 256 megabit or even higher. The integrated circuits devices like transistors, capacitors, and connections must be greatly narrowed accompanying with the advancement. At the same time, the region for forming the isolations must be greatly reduced.
The increasing packing density of the integrated circuits generates numerous challenges to the formation of the isolation region in the semiconductor manufacturing process. The isolation region need to be formed within smaller area without influencing the isolating characteristics and the operations of the devices and the circuits. The present technology research focus mainly on fabricating highly reliable and densely arranged isolation region for the sub-micrometer or even smaller semiconductor devices.
The shallow trench isolation (STI) which forms isolations by etching and refilling trenches, is a widely employed technology for densely packed circuits. The STI replaces the conventional LOCOS (local oxidation of silicon) in several ULSI applications. The STI is used in the isolation of devices within the same tub, of bipolar devices, and of n-channel and p-channel devices. The STI is also employed in forming the trench-capacitors for DRAMs.
Unfortunately, the conventional STI has a serious problem of damaging the operations of the active devices with a reduced threshold voltage. A problem called kink effect arises from an anomalous channel current turn-on at subthreshold regime. The kink effect is caused by enhanced electrical field at the edge of the active region. It was found that the STI corner oxide recess is a major issue. Referring to FIG. 1, a cross sectional view of a conventional STI structure is shown. A trench isolation structure 10 is formed in a semiconductor substrate 12. A liner oxide layer 14 is formed conformably within the trench as an insulation layer. A filling layer 16 like a CVD (chemical vapor deposited) oxide layer is filled within the trench. A conductor layer 18, generally a row of polysilicon gate structure, is formed over the semiconductor substrate 12 and crossing over the trench isolation structure 10. A gate oxide layer 20 is insulated between the semiconductor substrate 12 and the conductor layer 18.
A recessed shape at the top of the CVD oxide 16 is formed during the planarization process in the formation of the trench structure 10. A portion of the trench structure 10, circled by the region 22, is enlarged in FIG. 2. A convex edge 12a is found in adjacent to the trench structure 10. The thickness of the liner oxide layer 14 is much thinner at the convex edge 12a because of the sharp angle and the reduced oxide growth rate under the crystalline orientation effect of the semiconductor substrate 12. Under the effect of the thinner oxide, the electric field at the convex edge 12a is much stronger then other portions of the semiconductor substrate 12. Thus the current of the device under operation is turned on with a lower voltage at the convex edge 12a. The channel turn-on at the subthrehold voltage is called the "kink effect". The kink effect reduces the threshold voltage and induces the current at undesired locations. The operation of the active devices are influenced. Furthermore, the recessed shape at the top of the CVD oxide 16 can enhance the problem. The electric field is enhanced at the convex edge 12a by the lower and closer location conductor layer 18 which formed over the recessed region. Thus the threshold voltage is further reduced.
To fabricate the future integrated circuits with high integration density of the devices, a solution to the kink effect is required. A trench isolation technology without reducing the threshold voltage and without damaging the operation of the devices is needed.